It looks a lot like the end. Just as viewers of a movie franchise know the formula so well, they can tell when the final reel is under way, so we're getting used to the way Arab revolutions unfold – and sense that the signs point to a denouement in Syria. The key moment came this week with the assassination of four members of the Assad ruling clique by a still-mysterious bomb. The rumour mill promptly generated two storylines whose equivalents had been heard in the final days of the ancien régimes in Tunisia, Egypt and Libya: the president's wife had fled abroad (to Russia) and the president himself was nowhere to be seen. Assad surfaced eventually, but when the dictator has to appear on TV just to prove he's alive, the end seems imminent.

Of course, there could be a twist to this sorry tale. Bashar Assad's more pessimistic opponents recall the Desert Storm momentum that meant Saddam Hussein's days were surely numbered in 1991 – only for those days to number another 12 years. The Damascus regime still has a mighty arsenal and, in Russia and Iran, two powerful allies. It could cling on, fighting a sectarian civil war that could last months or even, as in Lebanon in the 1970s, years.

But let's assume that the House of Assad is crumbling. Its fall will obviously transform Syria, a country that has lived under the boot-heel of that clan for four decades. But it will also radically affect the wider region. Syria, which borders Jordan, Iraq, Lebanon, Turkey and Israel, does not keep itself to itself. As one former Obama official says: "Syria won't implode; it will explode." Put simply, the battle for Syria is a battle for the entire Middle East.

Take the most probable consequence of Assad's removal, a round of revenge killings perpetrated by Syria's Sunni majority on Assad's Alawite community and their Christian allies. They will be seeking vengeance, not only for the thousands slain in the current uprising, but for a history of brutality that includes the slaughter of up to 20,000 in Hama in 1982, the last time an Assad faced popular protest.

If that kind of sectarian violence erupts, don't expect it to stay confined to Syria. Even if the killing does not spill over the borders, then Syrians themselves will, joining the 125,000 who have already fled as refugees. And that's without Syria becoming the site of an all-out proxy war, with Saudi Arabia backing the rebels and Iran lining up behind the pro-Assad forces.

The west will not stay aloof for long. (Some say it is already involved, tacitly backing Saudi and Qatari arms shipments to the rebels.) Strikingly, the talk in the last 48 hours has shifted from direct intervention – for which there were few takers – to an international peacekeeping force to be dispatched after Assad's exit. Former CIA official Bruce Reidel, who led President Obama's 2010 review of US policy on Afghanistan and Pakistan, today proposed just such a force, noting the paradox that one of its first tasks would "be to protect the Alawite community and its allies from vengeance". Both the US and Israel are also anxiously eyeing Syria's supply of chemical and biological weapons, now said to be unlocked and on the move, fearing Assad may choose to go down in a lethal blaze glory.

So this is no domestic matter affecting Syria alone. The most immediate impact will be felt by Iran, which stands to lose not only its pivotal Arab ally but also the gateway Syria has long provided to Iran's proxy force in Lebanon, enabling Tehran to put upwards of 40,000 rockets in the hands of Hezbollah. Without Syria, Iran will lose that vital strategic bridgehead into the Arab world (even if, thanks to the US-led invasion in 2003, it can now count Iraq as friendly). But it goes deeper than that.

Iran's previous claim to lead an "axis of resistance", inspiring Syria, Hezbollah and Hamas to stand firm against the US and Israel, will be silenced. "It was losing that already," says Middle East analyst Daniel Levy, noting both Hamas's defiance of Tehran to side with the Syrian rebels and an Arab spring that is rendering obsolete Iran's previous claim that the Arab nations were uniformly led by autocrat-puppets of the US. Just six years ago, during Israel's Lebanon war, the leaders of Iran and Hezbollah, although they are Shia, were popular heroes on the Sunni Arab street. That, says Levy, wouldn't happen in the sectarian climate of today.

The fall of Assad will do more than diminish Iran. It will mark the passing of an entire political culture in the region. For Assad is the last representative of a form that dominated the Middle East for half a century: that of the secular strongman, the dictator backed by a merciless intelligence apparatus, what Chatham House's Nadim Shehadi calls "a Stasi state, where everyone is watching everyone else".

What began with Nasser in Egypt – or even Atatürk in Turkey – will end with Assad: the regime that represses local and ethnic difference in the name of a nationalism centred cultishly on the leader. In its place, Shehadi says, will come at first the chaos of hundreds of new parties and an even greater number of "mediocre politicians". But eventually, he hopes, it will pave the way for a post-dictatorship Middle East, a place where rulers stand or fall not on their ability to exploit problems as moves in a geopolitical power game, but to solve them instead.

It's an optimistic prognosis for a region that could be about to explode in bloody violence. But the fate of Syria will be decisive either way. If Assad holds on, then the Arab awakenings of 2011-12 will only ever have been a partial success. But if the Syrian rebels succeed, they will have achieved a sweeping victory. They will have effected a revolution without the full-blown foreign intervention required in Libya and more completely than in Egypt, where the security apparatus remains in place. That the revolt will have taken so long may even be a sign of strength, proving a depth and resilience that overnight insurrections elsewhere could not match.

Syria is on the brink. What will follow is not clear, given the mixed and divided nature of the opposition. This much we know: on the fate of Syria hangs the fate of the earth's most combustible region.